The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the transmission ratio at a minimum transmission ratio.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependence on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to decide the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on pulleys in order to transmit the output of the engine.
At the start of the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (to upshift). The transmission ratio is automatically and continuously reduced at a speed which is decided by line pressure, pressure of oil supplied to the servo device of the drive pulley, and actual transmission ratio. In such a system, the speed of changing of transmission ratio up to a desired transmission ratio is not controlled in accordance with driving conditions. Accordingly, hunting or overshoot of transmission ratio occurs, which causes the driveability of the vehicle to reduce.
Japanese Patent Laid Open No. 59-217048 discloses a system for controlling the transmission ratio changing speed e. Various values of the speed are stored in a look-up table and proper speed is derived from the table in accordance with engine speed and vehicle speed. The derived transmission ratio is used for deciding the control signal for the transmission ratio. However, if the viscosity of oil varies with change of temperature thereof or the variances of control valves caused by manufacturing tolerance and/or by changes with time, transmission ratio can not be properly controlled to a desired transmission ratio. As a result a deviation between the desired ratio and the actual ratio remains in a steady driving state.